On the formation of LaFe5Hn

We explore the formation of a LaFe5Hn hydride by means of density functional theory. Enthalpies of formation ΔH with respect to the elemental metals and H2 are calculated for various hydrogen configurations in four prototype crystal structures. We find ΔH < 0 in many cases, suggesting the existence of LaFe5Hn, as does Miedema's semi-empirical model. ΔH is a minimum for the LaFe5H7 stoichiometry with hydrogen occupying the 4e, 8g, and 16m sites in the orthorhombic Cccm structure. Phonon dispersion relations and elastic constants computed for that structure exhibit no anomalies, demonstrating vibrational stability. Similar results for LaFe5 indicate the possible formation of that compound under pressure.